Author's Note: This was originally uploaded on an old Tumblr account I had; I came across it again recently, and decided it was still something I liked well enough, so I'm posting it on here. It is essentially my take on old folktales concerning the origin of fairies (particularly Christianized versions of those tales) and interpreting how they would fit in the Grimm universe. I hope you enjoy it! I'm tempted to continue this story in some form... We shall see...
Since the beginning of the world, Wesen have lived alongside Kehrseite. The two races have had a complicated relationship, with the Wesen once being worshipped as gods by the Kehrseite, to the Kehrseite persecuting the Wesen. But despite all of this, there was a time when Wesen and Kehrseite were one and the same.
It is said that at the dawn of time, after the Lord Almighty had cast Adam and Eve out of Paradise, He came to visit them in their new home, in the land that the two had settled in. When He arrived, Adam greeted Him, and Eve brought out her children to be presented to God. God looked at all the children Eve had- all of them beautiful- and finally He asked "Do you have any more children?"
"None," Eve said. But that was far from the truth. Actually, Eve had many more; but when they were born, something was wrong with them. They were more like animals- foxes, wolves, beavers, mice, and rats, among others- and even though they walked and talked like the other children, Eve was embarrassed by them. No matter how hard she tried, Eve couldn't make them look like the rest of her children. Before God came to visit, Eve hid the other children behind the house, hoping God would not notice.
But God sees all, and knew what Eve had done. Before he left at the end of the day, he turned to the two and said to them "What you have hidden from God, God will also hide from the world." The two did not understand what that meant, but when Eve came to fetch the other children, she found to her surprise that they now looked like her beautiful children. Eve rejoiced, but her children were hurt by her treatment of them. In actuality, they were still like animals; as they discovered later, they could still turn from human to animal at different times, like taking off and putting on a coat. But so hurt were they by how their mother had treated them, that they told their children to never show their true nature to the other children of Adam and Eve. And so the beautiful children became the Kehrseite, and the ugly children became Wesen. And from then on, they took their curse to their graves, as a reminder of the shame Eve had, and also of what happens when one tries to hide something from the Lord.